neurons and nerve cell/sensory news

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How the brain interprets electrical impulses sent by neurons

News-Medical.net

Thursday, 3-Jun-2004

University of California, San Diego neurobiologists have uncovered

evidence that sheds light on the long-standing mystery of how the brain

makes sense of the information contained in electrical impulses sent to

it by millions of neurons from the body.

In a paper published this week in the early on-line version of the

journal Nature, a UCSD team led by Massimo Scanziani explains how

neurons, or nerve cells, in the brain sort out information before

deciding how to respond. The paper will appear in a forthcoming print

issue of Nature.

Light, sound and odors, for example, are transformed by our sensory

organs into a code made of series of electrical impulses that travel

along neurons from the body to the brain. Information about the onset

and the intensity of a stimulus is thought to be sent to the brain by

the timing and frequency of these electrical impulses. How information

is sorted by the brain has been an open question. The group discovered

that different neurons in the brain are dedicated to respond to specific

portions of the information.

“Our work shows that deciphering the enormous amount of information that

is conveyed to the brain at any time-point is a matter of division of

labor between specialized neurons,” explains Scanziani, an assistant

professor of biology. “Each neuron literally ’picks’ the type

information it is supposed to process, that it is competent for. Very

much like each musician in an orchestra only reads that part of the

score of a symphony that was written for his or her own instrument.”

Because they needed to see and record electrical impulses from

individual nerve cells, the researchers used slices of rat brain, which

when bathed in an appropriate solution can be kept alive under a

microscope. To mimic incoming information, the first author on the

paper, Frédéric Pouille, a postdoctoral fellow in Scanziani’s

laboratory, provided an electrical stimulus—analogous to the score in

Scanziani’s analogy—and then monitored which nerve cell read which part

of the information. Pouille and Scanziani found some nerve cells that

were only responsive to the first impulse that arrived, while other

nerve cells only responded to multiple electrical impulses arriving at

certain frequencies.

“While some neurons only responded to the onset of each package of

information, which, in other words, means: Hey, something just arrived,

other neurons actually looked into the package and played the notes,”

says Scanziani.

Each of these specialized brain neurons has a highly branched structure

where many neurons carrying sensory information can form connections. At

any moment, each of these specialized brain neurons might be receiving

multiple messages from multiple sources, but is only selectively

responding to certain information about the timing or frequency of the

impulses it is receiving.

Why is the timing of information so important? Visual, tactile and

auditory information needs to be synchronized. If it were not, then one

might, for example, perceive someone’s lips move before hearing the

words being spoken—like a badly dubbed foreign film.

The brain also needs to know how intense a stimulus is because intensity

will influence what action needs to be taken. For example, an

uncomfortable shoe will become more and more difficult to ignore as your

foot develops a blister. As the blister develops, the interval between

subsequent electrical impulses arriving at the brain would decrease; in

other words, their frequency would increase. Scanziani speculates that

there might even be an “alarm neuron” in the brain that responds to high

frequency electrical impulses by triggering the appropriate muscle

response to escape the stimulus.

“This study advances our understanding of how the brain reads a code

made of identical electrical impulses, in order to produce a coherent

perception of the world,” he says. “Deciphering the language of the

brain will help us understand the neuronal basis for sensation and

cognition and their associated disorders.”

In their paper, the UCSD researchers also determine a chain of

physiological mechanisms working in concert to allow these brain neurons

to selectively respond to a specific pattern of incoming electrical

impulses. Communication across the connections between neurons is

usually chemical rather than electrical. The researchers found that the

differences in the way the individual brain neurons released and

responded to these chemicals could explain their differing responses to

incoming information.

Scanziani and Pouille’s experiments focused on the hippocampus—a region

of the brain known to be important in learning in memory. But they

believe that other regions of the brain may also use the same principles

to sort information. However, the researchers point out that brain

slices are a simplified system, and more research is needed before they

will understand the finer details of this sorting.

“This is only part of the picture,” cautions Scanziani. “We are not

looking at the whole orchestra, maybe only the violins and the oboes.

But down the line we plan to look at further classes of nerve cells.”

The research study was initiated when Scanziani was an assistant

professor at the Brain Research Institute of the University of Zurich.

The work was supported by the National Institutes of Health and the

Swiss National Science Foundation.

http://www.ucsd.edu/